Okay first. A bit less of the bitza and more of the special if you don't mind me laddio!.
You have got me thinking with the thermal aspect of the sensor though. If it is thermal it must work on differential of temp between the top and bottom and not a specific temperature. Since the temperature of the petrol and for that matter the air could have quite a range given all circumstances, but if the sensor is fully submerged at least it will be all the same temp until the tip is exposed, when a difference might occur.
Looks like I'm going to have to get it wet..